The present invention relates in general to energy directing structures and assemblies, such as antenna reflector architectures, and is particularly directed to a new and improved support configuration for an energy directing surface, such as an RF reflective mesh, having an arrangement of ties and cords that are attached to and placed in tension by an inflated radial, truss-configured support structure, that facilitates compact stowage and stabilized deployment, and is therefore especially suited for spaceborne applications.
As described in the above-referenced ""294 patent, among the various conventional antenna assemblies that have been proposed for airborne and spaceborne applications are those which employ an inflatable medium, that may be unfurled from its stowed configuration to realize a ""stressed skin"" type of reflective surface. In such configurations, non-limiting examples of which are described in U.S. Pat. Nos. 4,364,053 and 4,755,819, the inflatable structure serves as the reflective surface of the antenna; namely, once fully inflated, the material is intended to assume and retain the desired antenna geometry.
Unfortunately, using the inflatable structure per se as the antenna surface creates several problems. First, the accuracy of the geometry of the antenna depends upon how faithfully the shape of the inflatable medium matches the antenna geometry, and also how well the shape of the inflatable medium can be maintained. Should there be (and there can expected to be) a change in the shape of the inflatable membrane, such as due to a change (most notably a decrease) in inflation pressure over time, the corresponding change in the contour of the inflatable structure will necessarily change the intended antenna profile, thereby impairing the energy gathering and focussing properties of the antenna. Although this inflation pressure decrease problem can ostensibly be addressed by the use of an auxiliary supply of inflation gas, it does not circumvent other causes of inflatable membrane distortion, such as, but not limited to, temperature and aging of the material, and particularly the fundamental ability of the inflated membrane to accurately produce the geometry of the antenna reflector.
In accordance with the invention described in the above-referenced ""294 patent, this inflation dependency problem is obviated by means of a hybrid antenna architecture, that effectively isolates the geometry of the antenna""s reflective surface from the contour of the inflatable support structure, while still using its support functionality to deploy the antenna. For this purpose, rather than make the reflective surface geometry of the antenna depend upon the ability to maintain a prescribed pressure, the inflated membrane is employed simply as a deployable ""tensioning"" attachment surface. The inflatable tensioning membrane may support the tensioning tie/cord arrangement and the adjoining antenna surface either interiorly or exteriorly of the inflatable membrane.
FIG. 1 (which, except for the reference numerals corresponds to FIG. 2 of the ""294 patent) is a cross-sectional view of an exterior support embodiment of this hybrid antenna architecture. The hybrid structure of FIG. 1 is taken through a plane that contains an axis of rotation AX. A generally parabolic reflective surface 10 of the antenna is made of a lightweight, reflective or electrically conductive material, such as, but not limited to, gold-plated molybdenum wire or woven graphite fiber. This surface is also rotationally symmetric about the axis AX, passing through an antenna feed horn 12.
The reflective surface 10 is attached by a tensioned cord and tie arrangement 20 to the exterior surface 31 of a generally toroidal or hoop-shaped inflatable support structure 30, which is also rotationally symmetric about the axis AX. The inflatable support structure 30 for the tie and cord arrangement 20 is joined to a support base 40 (e.g., a spacecraft) by way of a rigid truss attachment structure 50, that is formed of plurality of relatively stiff stabilizer struts or rods 51, also rotationally symmetric about the axis AX.
The inflatable hoop 30 may comprise an inflatable laminate of multiple layers of sturdy flexible material, such as Mylar. For deployment, the hoop 30 may be inflated through a valve 32, which may be located at or adjacent to its attachment to the truss 50, or the hoop may contain a material that readily sublimes into a pressurizing gas, that fills the interior volume 33 of the hoop 30.
The mesh reflector surface 10 is attached to the inflatable support structure 30 by means of tensionable ties 21 and cords 22 at perimeter attachment points 25, 27, distributed around the exterior surface 31 of the inflated membrane 30. This distribution of ties and cords is rotationally symmetric around the axis AX and is preferably made of a lightweight, thermally stable material, having a low coefficient of thermal expansion, such as woven graphite fiber. The hoop 30 is preferably inflated to a pressure greater than necessary to place the attachment cord and tie arrangement 20 at a minimum tension at which the reflective surface 10 acquires its intended shape.
This hybrid support structure enables the antenna surface to be maintained in a prescribed geometrical shape, that is independent of variations in the inflation pressure and shape of the hoop. Namely, the antenna is deployed and its geometry is fully defined once the inflatable hoop is inflated to at least the extent necessary to place the attachment ties and cords at their prescribed tensions. Preferably, the inflation pressure is above a minimum value that will accommodate pressure variations (drops) that do not allow the hoop to deform to such a degree that would relax or deform the antenna from its intended geometry.
In accordance with the present invention, the configuration of the inflatable tensioning structure for supporting the tensioning tie/cord arrangement and the adjoining antenna surface exteriorly thereof is that of an inflated arrangement of radially extending ribs and posts, that form radial truss elements with components of the tie/cord arrangement. These ribs and posts are readily collapsible to a compact configuration, to facilitate stowage and deployment, particularly for spaceborne applications. The inflatable rib structure contains a plurality of generally segment-wise curvilinear ribs that extend radially from an antenna boom through which a boresight axis of rotation passes, and to which an antenna feed horn is affixed.
For enhanced stability and rigidity, either or both of the radially extending curvilinear rib segments and the posts may be embedded with or affixed to stiffening elements, such as graphite rods or the like, oriented parallel to the intended directions of deployment. Distal ends of the rib segments and distal and base ends of the posts are connected to a truss-forming arrangement of collapsible cords, and circumferential cord segments. These cords are placed in tension by inflation of the ribs and act to stabilize the intended support geometry of the radial rib structure.
A reflective mesh surface is attached to the distal ends of the radial rib segments by a collapsible arrangement of tensionable ties and a set of radially extending backing cords. The backing cords are connected by tensioning ties to a plurality of attachment points distributed along the radial rib segments. Since the reflective mesh and its attachment ties and cords are collapsible, the entire antenna reflective surface and its associated tensioned attachment structure can be readily furled together with the inflatable radial surface in their non-deployed, stowed state. Each of these respective components of the support structure and the reflective surface readily unfurls into a predetermined geometry, highly stable reflector structure, once the ribs and posts of the radial support structure are fully inflated.